High performance permanent magnets, those having high energy products (BH)max, where B is the magnetic induction and H is the coercive field, can be broadly classified into three categories: rare earth-3d transition metal intermetallics (e.g., Nd2Fe14B, Sm1Co5 and Sm2Co17), AlNiCo (alloys composed primarily of iron with additions of aluminum, nickel, cobalt, copper, and sometimes titanium), and ceramic magnets (typically strontium-doped barium hexaferrites). Commercial permanent magnet applications include those for exerting attractive and repelling forces (e.g., magnetic separators, latches, torque drives, and bearings), for energy conversion (e.g., magnetos, generators, alternators, eddy current brakes, motors, and actuators), for directing and shaping particle beams and electromagnetic waves (e.g., cathode ray tubes, traveling wave tubes, klystrons, cyclotrons, and ion pumps), and for providing magnetic bias fields for a wide range of rf, microwave, and mm-wave devices (e.g., isolators, circulators, phase shifters, and filters). The magnets containing rare earth elements provide the highest energy products, (BH)max, but they are expensive and prone to corrosion, and pose severe cost limitations and supply chain challenges to commercial industries. Alternatively, AlNiCo and ceramic magnets have substantially lower (BH)max values but are significantly less expensive and more readily available from many sources. For that reason, AlNiCo and ceramic ferrite have captured substantial global permanent magnet market segments. The annual revenue generated by ceramic magnets is second only to that generated by high performance magnets of Nd—Fe—B.
However, very few additional developments in viable permanent magnet materials have occurred since the development of Nd—Fe—B in the early 1980s. Similarly, AlNiCo and ceramic magnets have not experienced significant improvement in permanent magnet properties for decades.
Improvements have come, though, to carbon-containing magnetic materials, which have many potential applications such as in high-density magnetic recording media, high resistivity soft magnetic materials, magnetic toner in xerography, and as contrast agents in high resolution magnetic resonance imaging. In previous work, researchers have focused on cobalt/carbide related materials that include carbon-coated magnetic-metal nanocrystallites (Wang et al., 2003), Co—C granular films (Lee et al., 2007; Konno et al., 1999; Wang et al., 2001), MnC (M=Fe, Co, Ni, Cu, n=1-6) nanoclusters (Black et al., 2004) and Co2C films (Premkumar et al., 20070. In those earlier works, the focus was placed on fabrication and application of carbon-related composites. The granular magnetic films, consisting of isolated particles suspended in a nonmagnetic host, were expected to produce low noise, high density magnetic media. The so-called core-shell nanoparticles constitute another form of nanocomposite. In the 1990s, McHenry et al. (McHenry et al., 1994) reported on the magnetic properties of carbon coated cobalt nanocrystallites. These nanocrystallites were proposed for applications ranging from recording media to emerging biomedical applications in imaging and cancer remediation therapies. Additional research has included theoretical and experimental studies of MnC (M=Fe, Ni, Co, etc.) clusters (Zhang et al., 2008), which are cage-like structures of transition metal containing carbon atoms that demonstrate unusual structural and chemical stabilities.